An ophthalmic lens blank generally has a first face with a pre-determined curvature and a second face, opposite the first face on which a desired surface contour is generated by a machining process. The overall process is generally referred to as “lens surfacing” and the overall object is to yield a finished spectacle lens wherein the first and second face curvatures cooperate to yield desired optical properties.
In prescription workshops, the following main process steps are usually carried out: Firstly, a suitable right and/or left ophthalmic lens blank is removed from a semifinished product store. The term “semifinished” is used to mean that the ophthalmic lens blanks, which are usually round or oval in plan view and have not yet been edged, have already been machined or in another way contoured on one of their two optically active faces. The ophthalmic lens blanks are then prepared for the blocking operation, namely by applying a suitable protective film or a suitable protective lacquer to protect the optically active face which has already been machined or contoured, i.e. the first face or blocking face.
The so-called “blocking” of the ophthalmic lens blanks then takes place. During this, the ophthalmic lens blank is joined to a suitable lens block, for example a lens block according to German standard DIN 58766. To this end, the lens block is firstly brought into a predefined position with respect to the protected first face of the ophthalmic lens blank, and then in this position the space between lens block and ophthalmic lens blank is filled with a molten material (normally a metal alloy or wax). Once this material has solidified, the lens block forms a holder or support for machining the second face of the ophthalmic lens blank. The lens block is grasped by chuck or other suitable coupling means during lens generation to provide in particular secure mounting to the profiling machine while avoiding damage to the lens.
Lens surfacing is carried out then using profiling machines which typically have a cutter of some type that is moved across the second face of the ophthalmic lens blank to give the second face its macrogeometry according to the prescription. The lens blank may be stationary or rotating during the cutting operation, depending on the particular profiling machine which is being used. Typical machining processes for surfacing ophthalmic lenses include single point diamond turning, diamond tool fly-cutting, milling, and grinding processes, applied depending on the lens material.
Usually fine machining of the ophthalmic lenses then takes place, in which the pre-machined second face of the respective ophthalmic lens blank is given the desired microgeometry. Depending on inter alia the material of the ophthalmic lenses, the fine machining process is divided into a fine grinding operation and a subsequent polishing operation, or includes only a polishing operation if a polishable second face has already been produced during the pre-machining stage.
Only after the polishing operation is the ophthalmic lens separated from the lens block (“deblocking”) before cleaning steps and possibly further refining steps are carried out, e.g. anti-reflection coating or hard coating of the ophthalmic lenses. The lens block accordingly remains on the ophthalmic lens (at least) for a number of machining operations and must remain reliably thereon during said operations.
Recently other bonding materials have been proposed for lens blocking in order to overcome certain disadvantages—the long time required for setting before the blocked lens can be safely released to subsequent processing operations, distortion problems with the lens caused by the heat associated with the molten material, and possible contamination of the lens, just to name a few—carried by the use of metal alloy or wax as the classical bonding agent. These other bonding materials include radiation curable materials.
In this connection document US 2005/0139309 A discusses the use of a UV light curable blocking material designed to reduce polymerization induced shrinkage. Although this development has been a significant step forward in allowing the use of UV curable materials for use in lens blocking, it still exhibits shrinkage of the order of 3%. This amount of shrinkage is generally not serious when the UV curable adhesive is used in a relatively thin (typically less than 3 mm) and uniform thickness; however it has serious limitations when thicker sections of adhesive are needed.
The shrinkage problem becomes even more apparent when blocking lenses having strongly non-uniform cross sections so that non-uniform sections of adhesive are needed. An example of this can be seen in FIG. 10 which shows a flat top bifocal lens blank 10 having a step discontinuity 12 between a bifocal segment 14 and a base curve 16, blocked on a lens block 18 by means of an UV cured adhesive 20. As is evident from FIG. 10 shrinkage can be a problem when the gap 22 between the bifocal segment 14 and the lens block 18 is significantly different than the gap 24 directly above the bifocal segment 14. In this instance the difference in shrinkage caused by the difference in thickness of the UV cured adhesive 20 can cause critical/unwanted distortion directly above the bifocal segment 14.
In an attempt to overcome the shrinkage problems remaining with the above UV light curable blocking material it has been proposed to use numerous (7 or 8) different block base curves to approximately match the conceivable lens base curves, and thereby keep the adhesive thickness sufficiently uniform to minimize residual shrinkage effects. FIG. 11 illustrates the prior art using numerous lens blocks 18 each comprising a lens mounting face 26 that has a pre-determined block curve, wherein the block curves of different lens blocks 18 differ from each other. In the example shown the lens blocks 18 comprise essentially spherical lens mounting faces 26 that have block curves of a) 0.5 diopters, b) 2 diopters, c) 6 diopters, and d) 10 diopters, respectively, generally matching the lens curves, i.e. the respective curvature of the blocking faces 28 of the blocked lens blanks 10. It is to be noted that only four different lens blocks 18 are shown in FIG. 11 to simplify the illustration; actually however this system used seven or eight different lens blocks, as mentioned before. As becomes apparent from FIG. 11 the UV cured adhesive 20 between lens blank 10 and lens block 18 in each case has a relatively thin and substantially uniform thickness so that there is no critical distortion due to shrinkage of the UV cured adhesive 20.
However the approach of using numerous different block base curves does not help in the situation shown in FIG. 10. A further problem associated with multiple block base curves is the management of these numerous curves. The proper lens blocks initially need to be selected to match the lens curves, then, after deblocking, (assuming the lens blocks are reused), they need to be properly sorted according to curve, and stored in separate holding/dispensing containers. Using multiple block curves thus adds to the complexity of the manufacturing process, increases the probability of error, and as a result increases the cost of lens manufacturing.